Laser fluence compensation of a curved surface

ABSTRACT

A laser system and techniques which compensate for laser fluence drop off or losses of irradiation as an ablating laser beam is traversed on a curved surface (e.g., on corneal tissue). The disclosed ablating laser system and techniques compensate for fluence differentials from pulse-to-pulse by adjusting an appropriate parameter of a laser beam. In the preferred embodiment, the number of pulses imparted in the periphery, the size or shape of the ablating laser beam is adjusted with, e.g., a variable aperture placed in the beam delivery path, by changing a magnification of relay optics in the beam delivery path, or by increasing a number of ablation spots in peripheral portions of an ablation zone as compared with the number of ablation spots in a central portion of the ablation zone. The fluence is compensated for using empirically measured or theoretical fluence correction factors given the angle of the laser beam, size and shape of the ablation spot, etc. In addition to magnification adjustment, the present invention also employs the technique of changing the size of the aperture that is imaged o the eye to provide uniform energy density (i.e., fluence) throughout the entire area of the irradiation site. These techniques are used independently or in conjunction to reshape the curvature of the eye to correct myopia, hyperopia, astigmatism or combinations thereof.

[0001] The present application claims priority from U.S. ProvisionalApplication No. 60/175,634, filed Jan. 12, 2000, entitled “Laser FluenceCompensation” to Michael L. Kliewer; and from U.S. ProvisionalApplication No. 60/196,290, filed Apr. 12, 2000, entitled “Laser FluenceCompensation of a Curved Surface” to Michael L. Kliewer, the entirety ofboth of which are expressly incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a laser surgery system. Moreparticularly, it relates to a non-contact laser ablation method andapparatus providing laser fluence compensation of a curved surface,especially a corneal surface.

[0004] 2. Background

[0005] The cornea and lens of an eye act in unison on light entering theeye to focus the incoming light onto the retina. When the refractivepower of the cornea and lens are optimized for the length of the eye, asharp image is focused on the retina. Myopia (nearsightedness) is theresult of blurred images caused when the focal point of the image islocated before the retina. Hyperopia (farsightedness) is the result ofblurred images caused when the focal point of the image is behind theretina. Astigmatism is a unique refractive error that causes reducedvisual acuity and produces symptoms such as glare, monocular diplopia,asthenopia and distortion and occurs when the focus from tangentiallight rays are at a different point than the focus of the sagital lightrays.

[0006] Vision acuities result from refractive errors from the corneal ofthe eye and the lens within the globe of the eye. For example,nearsightedness, or myopia is a result of the shape of the cornealmembrane being too steep.

[0007] One popular technique for correcting vision acuities is reshapingthe cornea of the eye. The cornea is chosen for modification beforeother components of the eye because it is the strongest refractingcomponent of the eye and is accessible without interoccular surgery. Asan example, the cornea of a patient with hyperopia, or farsightedness isrelatively flat resulting in a large spherical radius of the cornea. Aflat cornea creates an optical system that does not correctly focus theviewed image onto the retina of the eye but in fact the focal point isbeyond the surface of the retina. Hyperopia can be corrected byreshaping the eye to decrease the spherical radius of the cornea. In thecase of correcting hyperopia, corneal tissue is typically not removed atthe center of the cornea but is removed deeply at the periphery of thecornea.

[0008] As another example, to correct myopic effects of an eye,procedures are performed which effectively increase the radius of thecornea. In this case, the corneal surface is removed deeply at itscenter and slightly at its periphery.

[0009] In another example, such as the case of the correction ofastigmatism (e.g., myopic astigmatism), the surface of the cornea isremoved deeply at its center but only along a certain axis and slightlyat its periphery. The resulting shape of the cornea is that of acylindrical convex lens.

[0010] Changing ablation patterns on the cornea performs the variousvision corrections. Use of an ablating laser beam for removing thesurface of the cornea to correct ametropia of any sort requires preciseadministration of the laser beam. Optical systems are commonly used tocontrol or condition an ablating laser beam exiting from a laser sourceprior to impingement onto a corneal surface.

[0011] A common ablation laser system scans and pulsates an ablatinglaser beam across a corneal surface. Typically, the laser source isfixed in location with respect to the patient's eye, as is the patient.To remove corneal tissue throughout a given ablation pattern, theablating laser beam is typically directed across the corneal surfacewith the use of scanning mirrors.

[0012] However, as is appreciated by the present invention, ablation ofa curved surface introduces several dynamics that are typicallyunaccounted for in conventional laser ablation systems.

[0013] For instance, as shown in FIG. 4A, when the ablating laser beamis ablating a spot on the cornea directly below, a ‘direct’ hit on thecornea causes a maximum amount of energy absorption and transfer betweenthe laser beam and the corneal tissue being treated. This is because anormal or perpendicular angle θ is formed between the laser beam and thesurgical plane. However, as the angle of the laser beam with respect tothe surgical plane changes from 90° as shown in FIG. 4B, less energyfrom the laser beam transfers to the corneal tissue, resulting inchanging depths of ablation across the ablated curved surface.

[0014] Generally speaking, the eye is a spherical surface as depicted inFIGS. 4A and 4B, and the angle of incidence of the scanning laser beamon the eye varies as the with respect to distance from the apex of theeye. FIGS. 4A and 4B illustrate that the farther the apex of the laserbeam is from the center of the targeted curved surface 10, the greaterthe angle of incidence 2θ′ of the laser beam due to the curved surface10. This is especially true for small beam, scanning laser ablationsystems, although it is also true for broad beam systems. The broad beamsystems must compensate for the loss of power transferred to the corneaat the periphery.

[0015]FIG. 4B is illustrative of the reflection of additional laserenergy off the curved irradiation site due to the enlarged spot sizebeing projected onto the cornea.

[0016] As appreciated by the present inventor, as an ablating laser beamspot traverses a corneal surface, it tends to become elliptical on thecurved surface, and assumes a larger area. Fluence is defined as energyover area, or energy density. Thus, as the laser beam angles steeper andsteeper (i.e., further from a normal to the surgical plane) at the edgesof a larger and larger ablation pattern, the fluence decreases. This isappreciated to result in ablation depths toward the edges of theablation pattern which are less than the expected depth, and less thanthe ablation depth at a central portion of the ablation pattern at apoint directly below a normal angled laser beam.

[0017] The increased depth per pulse in the central portion of theablation pattern as opposed to the peripheral portions of the ablationpattern often cause the resulting shape to be non spherical and willchange the prolate nature of the cornea. The non-uniform removal oftissue (e.g., corneal tissue) can produce an irregular corneal outersurface and may even prevent proper healing.

[0018] There is a need for an ablation apparatus and method, whichprovides greater control and uniformity of the depth of ablation acrossan ablation pattern on a curved surface such as a corneal surface.

SUMMARY OF THE INVENTION

[0019] In accordance with the principles of the present invention, anablation laser system having variable fluence comprises a laser source,and relay optics for delivering a laser beam from the laser source to atarget surface. The number of pulses is increased in the periphery tocompensate for the reduced ablation due to reduced fluence in thisregion.

[0020] In accordance with another aspect of the present invention, anablation laser system having variable fluence comprises a laser source,and relay optics for delivering a laser beam from the laser source to atarget surface. An ablation spot fluence adjuster adjusts a fluence ofan ablation pulse on the target surface.

[0021] In accordance with another aspect of the present invention, asystem for imparting ablating laser radiation onto a target curvedsurface comprises a laser source having an output laser beam, and avariable aperture device. A controller is operatively connected to theaperture to adjust the diameter of the laser beam. Relay optics producean image of the laser beam, and turning optics scan the image of thelaser beam across the target surface.

[0022] A method for providing laser radiation on a curved surface havinga desired fluence throughout in accordance with yet another aspect ofthe present invention comprises providing an ablating laser beam. Across-sectional shape of the ablating laser beam is set to a first size,with respect to a particular ablation spot of a particular ablationpattern on a particular layer of tissue to provide a given fluence levelfor that particular ablation spot. The ablating laser beam is scanned toanother ablation spot of the particular ablation pattern on theparticular layer of tissue, and the cross-sectional shape of theablating laser beam is re-adjusted to a second size different from thefirst size, with respect to another ablation spot, to maintain the givenfluence level for the ablation spot.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Features and advantages of the present invention will becomeapparent to those skilled in the art from the following description withreference to the drawings, in which:

[0024]FIG. 1 illustrates a laser surgical system including an ablationspot fluence adjuster and fluence correction factors, in accordance withthe principles of the present invention.

[0025]FIG. 2 is a graph representing exemplary correction factors usedby a spot fluence adjuster for index and fluence as the angle versustreatment radius changes, in accordance with the principles of thepresent invention.

[0026]FIG. 3 is a flowchart illustrating an exemplary process ofcompensating for laser fluence as a laser beam scans across an ablationpattern, in accordance with the principles of the present invention.

[0027]FIGS. 4A and 4B show a laser beam of a scanning laser ablationsystem as it places an ablation pulse in a central region of a targetsurface (FIG. 4A) and as it places an ablation pulse in a peripheralregion of a target surface (FIG. 4B).

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0028] The present invention recognizes the fluence differential of anablating laser spot across a curved surface, and provides apparatus andmethods to compensate for the decrease in fluence on the periphery of acurved ablated tissue surface, e.g., a cornea.

[0029] In accordance with the principles of the present invention, thefluence administered to an ablated surface is controlled and/or variedin relation to an angle between the impinging laser beam and the angleof the target surface at the relevant ablation spot.

[0030] In one embodiment, the size of the ablation spot image projectedonto a surgical plane is controlled to maintain a particular fluence asthe ablation spot scans across a target surface. The size of theablation spot image may be varied, e.g., by adjusting relay optics thatimage the aperture on the eye (i.e., by changing the magnification powerof the optics), or by simply changing the size of an aperture in thelaser beam delivery path.

[0031] In any event, in accordance with the principles of the presentinvention, the fluence of the ablating laser beam on the eye isincreased or decreased depending upon the angle of the laser beam and onthe angle of the surface being treated at the point of the ablation spotbeing contemplated. Given those angles, the actual size of the spot, andthus the actual fluence at a particular ablation point can be moreaccurately determined, and adjusted to meet a predetermined fluenceprofile. As a result, the fluence of ablation spots (particularly towardthe periphery of an ablation pattern) are actively controlled to beconsistent with expected results (e.g., to maintain a consistent fluencefor all ablation spots across an ablation pattern, resulting inconsistency, uniformity, and predictability throughout any givenablation pattern.

[0032] The compensation can also be calculated beforehand in a planningexercise. The new ablation pattern with compensation can then beimparted onto the cornea.

[0033] There are four exemplary approaches employed by the presentinvention to achieve the desired pulse-by-pulse fluence control. In afirst approach, the fluence of the beam image may be adjusted byadjusting the power of the laser beam. In a second approach, the size ofthe beam image impinged on the cornea may be adjusted to adjust thefluence. In a third approach, both laser power and beam size may beadjusted to achieve an appropriate beam image necessary for affectingthe desired refractive correction. In the fourth method, the number ofpulses incident upon the cornea are increased at the periphery in orderto compensate for the decrease in fluence and ultimately the decrease inablation depth.

[0034]FIG. 1 illustrates a laser surgical system including an ablationspot fluence adjuster and fluence correction factors, in accordance withthe principles of the present invention.

[0035]FIG. 1 depicts a block diagram of the ophthalmic laser system of apreferred embodiment. The laser source 20 is a basic laser, e.g., abasic, fundamentally ultraviolet laser such as an excimer laser. Thelaser source 20 produces a laser beam 25 which travels through anaperture 30 to relay optics 40 to produce an image of the beam 25′. Alaser beam image 25′ is coupled to a corneal surface 10 by the turning(scanning) optics 50. The turning optics are reflective or semireflective optical elements which change the axis of the laser beamimage 25′ in two dimensions from that of the source to the axis of thecorneal surface. The turning optics are reflective optics which mayinclude mirrors, transmission optics with a highly reflective coating,or reflective coatings on reflective optics. Suitable scanners includegalvanometers, mirror arrays, octagonal mirrors, etc.

[0036] A suitable controller 60 (e.g., a microprocessor, amicrocontroller, or a digital signal processor (DSP)) controls the relayoptics 40 and the aperture 30 with information provided by an opticaldetector 80. Data provided by an initial step of topography or wavefrontmay be used to provide the actual curvature and form the basis of acorrective procedure to be performed by the ophthalmic laser system. Ofcourse, the invention also relates to non-custom ablation systems andtechniques relying on more conventional refractive measurements of apatient's cornea, and presuming a given shape of the cornea.

[0037] The relay optics 40 are adjusted by control of the controller 60.The variable size aperture 30 may be adjusted to provide the appropriatebeam size and energy densities to cause a desired actual fluence on thecorneal membrane.

[0038] From a system standpoint, the invention can include a laser beamgenerating device or source 20 for producing a beam of radiation along apath, an aperture 30 for adjusting beam size (and thus fluence), and abeam imaging means comprising a relay lens system 40 for imaging adesired image of the laser beam onto the eye 10, and a spot fluenceadjuster with appropriate fluence correction factors relating tospecific angles of the laser beam and/or target surface.

[0039] The target surface may be any tissue surface having a curvedsurface, e.g., skin, cornea, etc. However, the preferred embodiment ofthe present invention relates to the reshaping of the cornea of an eye.

[0040] As above, in a first method of the present invention, the fluencemay be adjusted by adjusting the relay optics 40 for a greater or lesserenergy density. The relay optics 40 control the size of the laser beam.Since an image of the laser beam has been created by the relay optics40, the fluence levels can be controlled and made constant throughoutthe image of the beam. When the laser beam is condensed, the energydeposited on the surface 10 increases. Alternatively, when the laserbeam is expanded, the energy deposited on the surface decreases.

[0041] In a second technique, the energy density imparted on the cornealsurface 10 is adjusted by fixing the size of the image of the laser beamaccordingly. The size of the image of the laser beam is adjusted byadjusting the opening of the aperture 30 as indicated by the arrows inFIG. 3. By adjusting the size of the laser beam image as a function ofthe angle of incidence, a static energy density with the required beamprofile can be achieved. The size of the image of the beam may be madelarger or smaller by adjusting the aperture 30. This adjustment of theaperture may be carried out manually or by the microprocessor.

[0042]FIG. 2 is a graph representing exemplary correction factors usedby a spot fluence adjuster for index and fluence as the angle versustreatment radius changes, in accordance with the principles of thepresent invention.

[0043] In particular, as shown in FIG. 2, the periphery portion of theablation pattern is represented by an increasing treatment radius. Ascan be seen in FIG. 2, the fluence is increased as the treatment radiusincreases (i.e., as the angle of the laser beam off-normal increases,and/or as the tangent of the ablated spot increases in angle). This isdue to the increased reflectivity of the cornea at angles greater than90 degrees.

[0044] The upper graph of FIG. 2 shows fluence variation with respect tothe radius of corneal eye tissue, and the lower graph of FIG. 2 showsfluence variation with respect to a curved test surface such as PMMA.

[0045] Of course, instead of maintaining a consistent fluence throughoutall ablation spots in a particular ablation pattern, the fluence mayinstead be controllably varied across the ablation pattern on apulse-by-pulse basis, consistent with a predetermined treatment.

[0046] Fluence control and variation over a single ablation pattern on asingle layer of tissue provides further and advanced ability tocustomize ablation treatments. For instance, irregularities on anindividual's cornea can and usually are patient specific, and a goal ofcustom ablation surgery is to remove those unique irregularities thatare detrimental to the vision of a particular patient. Topography,wavefront, or other tissue mapping techniques may be used to provideinformation relating to bumps, pits and other irregularities on thetarget surface, and fluence compensation at the relevant bumps or pitsmay be adjusted and actively controlled on an ablation pulse-by-ablationpulse basis to correct for such patient specific irregularities. Thus,in accordance with the principles of the present invention, customablation techniques can be augmented and/or implemented by activelycontrolling fluence across individual ablation patterns on individualtissue layers.

[0047] Apparatus and methods in accordance with the principles of thepresent invention are capable of correcting ametropia.

[0048] The laser source 20 may be any suitable ablation laser. Forinstance, an exemplary laser source 20 is a compact argon fluorideexcimer laser (at 193 nm) with repetition rate of (1-1,000) Hz having anoutput energy range of 0.1 mJ/cm² to 1 J/cm² with a pulse width of 1-100ns. Although the exemplary laser source 20 is an excimer laser, thelaser source 20 may be any suitable laser, e.g., liquid, gas orsolid-state laser source.

[0049] The laser source 20 may also include compact, optically-pumped(either flash-lamp or laser-diode pumped) lasers of Nd:YAG, Nd:YLF orthe self-frequency-doubling crystal of NYAB (neodymium yttrium aluminum)with pulse duration of 0.05-20 nanoseconds and repetition rate of1-10,000 Hz. It is known that this type of basic laser source 20 isavailable using a standard Q-switch or mode-lock, where the UVwavelength at 209-213 nm may be achieved by the frequency conversiontechniques using nonlinear crystals. The UV laser energy required forefficient ablation ranges from 0.01 mJ to 5 mJ. The basic laser alsoincludes an Er:YAG laser (at 2.94 microns) with repetition rate of(1-200) Hz, energy per pulse of (50-500) mJ, pulse duration of (50-400)nanoseconds and frequency-converted IR lasers of diode laser,optically-pumped Alexandrite or Li:SAF lasers, where efficient nonlinearcrystals may be used to convert the fundamental wavelength (770-880 nm)into its fourth-harmonic at the UV tunable wavelength of (193-220 nm)with energy of (0.01-5.0) mJ, repetition rate of (1-10,000) and pulseduration of (0.05-150) nanoseconds. Only two non-linear crystals areneeded in this case and overall efficiency is higher than that of thefifth harmonic generation which requires three nonlinear crystals.

[0050] The basic laser source 20 may also include ultrashort pulsedlasers, such as a commercialized mode-locked Ti:sapphire laser or othersolid-state lasers, with wavelength ranges of (750-1100 nm), repetitionrates of (0.01-100 MHz), energy per pulse of (0.01-100) microjoules, andpulse durations of (0.05-10) picoseconds where focused beam spot size of(0.05-0.5) mm is required to achieve the ablation threshold. A focusedspot size of (0.05-0.5) mm of the ultrashort pulsed lasers would beappropriate to achieve the tissue ablation and precise ablation profileproposed by the present invention. The above-described lasers may alsobe frequency-converted into UV ranges of (190-220) nm suitable forphotoablation.

[0051] The basic laser source 20 may also include a mid-IR (2.5-3.2microns) laser generated from optical parametric oscillation (OPO) usinga near-IR laser (such as Nd:YAG or Nd:YLF, flash-lamp or diode-pumped)as the pumping sources and KTP or BBO as the frequency conversioncrystals. The OPO laser has advantages over the Q-switched Er:YAG laser,including higher repetition rate (10-5,000 Hz) and shorter pulse width(1-40 n.s.). These advantages provide faster surgical procedure andreduced thermal damage on the ablated corneal tissue. Typical energy perpulse of the OPO laser is (0.1-10) mJ.

[0052] To further improve the controllability of the fluence of ascanning, ablating laser beam, an optical feedback mechanism may beimplemented and used in conjunction with the fluence control apparatus(e.g., aperture control and/or relay optics control) to maintain adesired spot size as the spot angles. The optical detector could alsomonitor the fluorescence of the cornea since the fluorescence isdirectly related to the fluence imparted.

[0053] The optical feedback mechanism may include an optical detector 80to provide a feedback path between the target surface and the controller60. The optical detector 80 provides real time images or fluorescenceintensities allowing measurements of the imaged laser beam 25′ on thecorneal surface 10. This data is then gathered by the controller 60 fordetermination of spot area, and thus fluence (given a fixed laser beampower) for evaluation and control of the beam image 25′ impinged ontothe corneal surface 10. Of course, if the power level of the laser beamis varied on a spot-by-spot basis during the procedure, the fluencecalculation may be determined appropriately.

[0054] Preferably, the optical detector 80 is a photodetector sensitizedto the particular wavelength of the ablation spot (e.g., to view 193 nmlaser light). Thus, the actual size of the ablation spot can be viewedby the optical detector 80, and processed by the controller 60. The spotfluence adjuster 100 may determine a desired fluence level based on themeasured size of the spot or fluorescence intensity, but moreparticularly based on the angle of the laser beam and/or target surface.The appropriate mechanism (e.g., a controllable variable aperture 30and/or a magnification or other characteristic of the relay optics 40)is adjusted under the direction of the controller 60 to arrive at adesired, actual fluence for that or a subsequent ablation spot.

[0055] Adjusting fluence on a pulse-by-pulse basis, or at least within asingle ablation pattern on a single layer of tissue, facilitates a moreprecise corrective refraction procedure than is conventionallyavailable.

[0056] The relay optics 40 may include zoom up collimators, anamorphicprisms, and the like. The relay optics 40 may be translated under thecontrol of the controller 60. By translating the relay optics 40, thesize of the image of the beam can be used to provide adjustable fluencelevels on the ablation zone of the cornea 10.

[0057] For instance, as in the correction of myopia, which requires aphotoablation scheme of more radiation toward the center and less aroundthe periphery of the corneal surface, the relay optics 40 may be movedaccordingly to produce an image of the laser beam having an energydensity profile of more concentrated energy density toward the center ofthe ablation zone and having less concentrated energy around theperiphery. Herein, the size of the beam is maintained while the fluenceis adjusted.

[0058]FIG. 3 illustrates an exemplary process for controlling orcompensating fluence as a laser beam scans across an ablation pattern ona layer of target tissue, in accordance with the principles of thepresent invention.

[0059] In particular, as shown in FIG. 3, an ablating laser beam isdirected along a beam path, and the energy density (i.e., fluence) isadjusted as the beam scans across a targeted ablation surface.

[0060] The present invention has particular application for customablation. For instance, prior to surgery, corneal topography orwavefront may be performed to collect surface feature or total eye datarelating to a particular patient's eye. The topography aides indetermining the ablation pattern as well as identifying anyirregularities on the corneal surface, as is otherwise know in the art.

[0061] Based on a given ablation pattern (which may or may not bedetermined based on customized correction of the eye), a given ablationpattern is determined. In accordance with the principles of the presentinvention, the ablation pattern relates not only to the number andlocation of ablation spots, but also to the fluence level which is to bedelivered to each ablation spot.

[0062] Thus, customized topography and wavefront data may be processedby the controller 60 to determine a particular refractive correctivescheme. The refractive correction scheme will typically include theplanned ablation of a plurality of layers of tissue, each layer havingan ablation pattern associated therewith, and each ablation patternhaving a particular fluence level associated with each ablation spot.

[0063] To further refine the delivery of exact fluence levels at eachablation spot, the controller 60 may receive real-time spot size, laserbeam angle, and/or target tissue angle information from the opticaldetector 80.

[0064] The controller 60 then evaluates whether or not the appropriatebeam image has been produced on the corneal surface. If adjustment isnecessary, then the fluence of the laser beam may be adjusted in anappropriate manner, e.g., by adjusting the power level of the laser, byadjusting the attenuation level of the laser beam in a fast-actingattenuator, by adjusting a fast-acting variable aperture mechanism, byadjusting a characteristic of the relay optics, etc.

[0065] The optical detector can be placed at another location in thebeam delivery path, before the laser beam impinges on the targetsurface, and the fluence level can be adjusted before allowed to impingeon the target surface.

[0066] While the invention has been described with reference to theexemplary embodiments thereof, those skilled in the art will be able tomake various modifications to the described embodiments of the inventionwithout departing from the true spirit and scope of the invention.

What is claimed is:
 1. An ablation laser system having variable fluence,comprising: a laser source; relay optics for delivering a laser beamfrom said laser source to a target surface; and an ablation spot fluenceadjuster to adjust a fluence of an ablation pulse on said target surfaceon a pulse-by-pulse basis.
 2. The ablation laser system having variablefluence according to claim 1 , wherein: said ablation spot fluenceadjuster controls a variable aperture through which said laser beampasses.
 3. The ablation laser system having variable fluence accordingto claim 1 , wherein: said ablation spot fluence adjuster controls amagnification of said relay optics.
 4. The ablation laser system havingvariable fluence according to claim 1 , wherein: said systemsubstantially overlaps ablation spots within a single ablation patternon a single layer of target tissue.
 5. The ablation laser system havingvariable fluence according to claim 2 , wherein: said systemsubstantially overlaps ablation spots within a single ablation patternon a single layer of target tissue.
 6. The ablation laser system havingvariable fluence according to claim 1 , further comprising: a fluencecorrection table accessible by said ablation spot fluence adjuster, saidablation spot fluence adjuster selecting a fluence correction factorfrom said fluence correction table based on an actually measured area ofa contemporaneous ablation spot.
 7. The ablation laser system havingvariable fluence according to claim 2 , further comprising: a fluencecorrection table accessible by said ablation spot fluence adjuster, saidablation spot fluence adjuster selecting a fluence correction factorfrom said fluence correction table based on an actually measured area ofa contemporaneous ablation spot.
 8. The ablation laser system havingvariable fluence according to claim 1 , further comprising: a scanner toscan said laser beam across said target surface.
 9. The ablation lasersystem having variable fluence according to claim 2 , furthercomprising: a scanner to scan said laser beam across said targetsurface.
 10. The ablation laser system having variable fluence accordingto claim 1 , wherein: said laser beam is 193 nm when output from a basiclaser of said laser source.
 11. The ablation laser system havingvariable fluence according to claim 2 , wherein: said laser beam is 193nm when output from a basic laser of said laser source.
 12. The ablationlaser system having variable fluence according to claim 1 , wherein:said laser source is a basic laser having a fundamentally ultravioletoutput at an output window.
 13. An ablation laser system having variablefluence, comprising: a laser source; relay optics for delivering a laserbeam from said laser source to a target surface; and an ablation spotfluence adjuster to adjust an ablation spot fluence on said targetsurface by increasing a number of pulses per unit area in a periphery ofsaid target surface as compared to a lower number of pulses per unitarea in a central portion of said target surface.
 14. The ablation lasersystem having variable fluence according to claim 13 , wherein: saidsystem substantially overlaps ablation spots within a single ablationpattern on a single layer of target tissue.
 15. The ablation lasersystem having variable fluence according to claim 14 , wherein: saidsubstantial overlap is at least 50% overlap of adjacent ablation spots.16. The ablation laser system having variable fluence according to claim13 , further comprising: a fluence correction table accessible by saidablation spot fluence adjuster, said ablation spot fluence adjusterselecting a fluence correction factor from said fluence correction tablebased on an actually measured area of a contemporaneous ablation spot.17. The ablation laser system having variable fluence according to claim13 , further comprising: a scanner to scan said laser beam across saidtarget surface.
 18. The ablation laser system having variable fluenceaccording to claim 13 , wherein: said laser beam is 193 nm when outputfrom a basic laser of said laser source.
 19. A system for impartingablating laser radiation onto a target curved surface, comprising: alaser source having an output laser beam; a variable aperture device; acontroller operatively connected to said aperture to adjust saiddiameter of said laser beam on a pulse-by-pulse basis; relay optics forproducing an image of said laser beam; and turning optics to scan saidimage of said laser beam across said target surface.
 20. The system forimparting ablating laser radiation onto a target curved surfaceaccording to claim 19 , wherein: said variable aperture device adjusts acircular diameter of said laser beam.
 21. The system for impartingablating laser radiation onto a target curved surface according to claim19 , wherein: said variable aperture device adjusts an ellipticaldiameter of said laser beam.
 22. The system for imparting ablating laserradiation onto a target curved surface according to claim 19 , furthercomprising: an optical detector to provide real-time data of said laserbeam image as it impinges onto said target surface.
 23. The system forimparting ablating laser radiation onto a target curved surfaceaccording to claim 22 , wherein: said real-time data includes beam shapeand size.
 24. The system for imparting ablating laser radiation onto atarget curved surface according to claim 22 , wherein: said real-timedata includes fluorescence of cornea.
 25. The system for impartingablating laser radiation onto a target curved surface according to claim22 , wherein said optical detector comprises: a photo detector.
 26. Amethod for providing laser radiation on a curved surface having adesired fluence throughout, said method comprising: providing anablating laser beam; setting a cross-sectional shape of said ablatinglaser beam to a first size, with respect to a particular ablation spotof a particular ablation pattern on a particular layer of tissue toprovide a given fluence level for that particular ablation spot;scanning said ablating laser beam to another ablation spot of saidparticular ablation pattern on said particular layer of tissue; andre-adjusting said cross-sectional shape of said ablating laser beam to asecond size different from said first size, with respect to said anotherablation spot, to maintain said given fluence level for said anotherablation spot.
 27. The method for providing laser radiation on a curvedsurface having a desired fluence throughout according to claim 26 ,further comprising: monitoring an image of said laser beam on saidcurved surface; and determining an area of said laser beam on saidcurved surface on a pulse-by-pulse basis.
 28. The method for providinglaser radiation on a curved surface having a desired fluence throughoutaccording to claim 26 , wherein: said cross-sectional shape of saidablating laser beam is adjusted with a variable aperture mechanism. 29.The method for providing laser radiation on a curved surface having adesired fluence throughout according to claim 26 , wherein: saidcross-sectional shape of said ablating laser beam is adjusted bychanging a magnification of relay optics in a delivery path of saidlaser beam.
 30. The method for providing laser radiation on a curvedsurface having a desired fluence throughout according to claim 26 ,further comprising: determining a fluence of an ablation spot on apulse-by-pulse basis.
 31. The method for providing laser radiation on acurved surface having a desired fluence throughout according to claim 26, further comprising: initially performing a topographical and/orwavefront measurement of said curved surface; and adjusting a fluence ofsaid ablating laser beam based on an irregularity on said curved surfaceidentified in said measurement.
 32. Apparatus to provide laser radiationon a curved surface having a desired fluence throughout, comprising:means for providing an ablating laser beam; means for setting across-sectional shape of said ablating laser beam to a first size, withrespect to a particular ablation spot of a particular ablation patternon a particular layer of tissue to provide a given fluence level forthat particular ablation spot; means for scanning said ablating laserbeam to another ablation spot of said particular ablation pattern onsaid particular layer of tissue; and means for re-adjusting saidcross-sectional shape of said ablating laser beam to a second sizedifferent from said first size, with respect to said another ablationspot, to maintain said given fluence level for said another ablationspot.
 33. The apparatus to provide laser radiation on a curved surfacehaving a desired fluence throughout according to claim 32 , furthercomprising: means for monitoring an image of said laser beam on saidcurved surface; and means for determining an area of said laser beam onsaid curved surface on a pulse-by-pulse basis.
 34. The apparatus toprovide laser radiation on a curved surface having a desired fluencethroughout according to claim 32 , wherein: said means for setting andsaid means for re-adjusting comprise a variable aperture mechanism. 35.The apparatus to provide laser radiation on a curved surface having adesired fluence throughout according to claim 32 , wherein: said meansfor setting and said means for re-adjusting comprise variablemagnification optics in a delivery path of said laser beam.
 36. Theapparatus to provide laser radiation on a curved surface having adesired fluence throughout according to claim 32 , further comprising:means for determining a fluence of an ablation spot on a pulse-by-pulsebasis.
 37. The apparatus to provide laser radiation on a curved surfacehaving a desired fluence throughout according to claim 32 , furthercomprising: means for initially performing a topographical and/orwavefront measurement of said curved surface; and means for adjusting afluence of said ablating laser beam based on an irregularity on saidcurved surface identified in said measurement.
 38. A method forproviding laser radiation on a curved surface having a desired fluencethroughout, said method comprising: setting a fluence rate of anablating laser beam to a first number of ablation spots per unit areawith respect to a central region of said curved surface; scanning saidablating laser beam in a central portion of said curved surface;re-setting said fluence rate of said ablating laser beam to a secondnumber of ablation spots per unit area higher than said first number;and scanning said ablating laser beam in a peripheral portion of saidcurved surface.